The performance of chip multiprocessors (CMPs) is governed not only by the performance of each processor core but also by their communication efficiency. As more and more processor cores are being integrated in one chip, traditional electrical interconnects have gradually become an obstacle to improving the performance of CMP because of their relatively low bandwidth, large power consumption and high delay^[1-3]. Optical interconnects are a promising solution to this problem.

Several topologies for photonic networks-on-chip (NoC) have been widely investigated, including Mesh, Fat-Tree, Clos, and Crossbar^[4-7]. Among them, the Mesh photonic NoC has received much attention because of its symmetric architecture, simple routing algorithm and good scalability. As one of the building blocks of the Mesh photonic NoC, the four-port optical router has received much attention. Compared with reconfigurable non-blocking optical router, a strict non-blocking optical router can be made more flexible for the network architecture and routing algorithm by adding more switching elements, and thus more routing choices. Various microring optical switches that are based four-port optical routers^[8-22] have been reported because they have a small footprint and low power consumption.

In this paper, we report a strict non-blocking four-port silicon optical router on a silicon-on-insulator (SOI) platform for a Mesh photonic NoC. It consists of eight microring optical switches tuning with thermo-optic effect. Furthermore, 8 × 40 Gbps data transmission utilizing wavelength division multiplexing (WDM) has been carried out through all of its 12 optical links in the C band.

Fig. 1(a) shows the schematics of a 16-core CMP with a 4 × 4 2D mesh photonic network linking 16 different processor cores. To ensure bidirectional transmission on the same link, each optical link has two unidirectional waveguides. A four-port optical router (seeFig. 1(b)) is located at the four edges of the Mesh photonic NoC, connecting four directions: East, West, South, and North. The OE/EO interfaces connect one port of the optical router to a local electronic block, while the other three ports are connected to three additional optical routers.

Fig. 2(a) shows the architecture of the four-port optical router. The eight microring optical switches are denoted as S_j (j = 1, 2, ..., 8). As displayed inFig. 2(b), when the microring is on-resonance at its working wavelength, the optical switch S_j is in the “bar” state; and when the microring is off-resonance at its working wavelength, the optical switch S_j is in the “cross” state. We have denoted the microring optical switch in the “bar” state as S_j(B) and the one in the “cross” state as S_j(C). The optical link from input port P (East, West, South, and North) to output port Q is marked as Pi → Qo (e.g., the optical link from the East input port to the South output port is marked as Ei → So). The optical route from the East input port to the South output port is denoted as Ei → S₁(C) → S₃(C) → S₅(C) → S₈(C) → So.

The input and output port in the same direction (East, West, South and North) are associated to the same node, and the connection between them has no sense. Therefore, this four-port strict non-blocking optical router contains 12 effective optical links in total. These connections can be controlled by a routing algorithm (shown inTable 1), which enables four parallel optical links for arbitrary one-to-one interconnection from any of the four input ports to any of other three output ports simultaneously without contention at the working wavelength.

The optical router is fabricated at AMF’s silicon photonics foundry and the design is defined using 248-nm-deep ultra-violet photolithography. The waveguide exclusively supporting the fundamental quasi-TE mode is a rib type, which has a height of 220 nm, a width of 400 nm, and a slab thickness of 70 nm. An inductively coupled plasma etching process is adopted to etch the top silicon layer. To prevent optical loss caused by metal absorption, plasma-enhanced chemical vapor deposition (PECVD) is used to form a 1500-nm-thick silica layer on the silicon waveguide. Then, on a separate layer, titanium nitride with 220 nm thickness is sputtered, and titanium nitride heaters with 1µm width are fabricated by UV photolithography and dry etching. Via holes are carved following the deposition of a 300-nm-thick silica layer by PECVD. Eventually, aluminum pads are created.Fig. 3 shows a micrograph of the fabricated optical router, which has a footprint of 400 × 450µm².

The eight microring optical switches are constructed with identical parameters, resulting in the same resonance wavelength. Increasing the radius of the microring optical switch, for example, can improve its aggregate bandwidth, while simultaneously expanding its footprint. The radii of the microring are chosen to be 20µm as a tradeoff between device size and aggregate bandwidth.

Fig. 4 shows the measured transmission spectra of the microring switch element on its “cross” and “bar” states. The coupling loss between the device and the lensed fiber is about 2.5 dB/facet. It is easy to determine the insertion loss is 0.8 dB for the optical link I₂–O₁ at the working wavelength when the microring switch element is on the “cross” state. The insertion loss is 1.8 dB for the optical link I₂–O₂ at the working wavelength when the microring switch element is on the “bar” state. The extinction ratios of the optical links of the microring switch on its two statuses are larger than 17 dB at the working wavelength.

To verify its routing operation, three routing states of the optical router, which cover all its 12 optical links and 12 particular routing paths, are chosen to form an independent test scenario.Table 2 displays the states of the eight microring optical switches in the three routing states that were chosen. The letter “B” or “C” inTable 2 indicates that the associated microring optical switch is in the “bar” or “cross” states, which correspond to on-resonance or off-resonance at the working wavelength, respectively.

To characterize the spectral responses of the optical router and identify the operating wavelength, an optical spectrum analyzer and an amplified spontaneous emission source are used. We measure the transmission spectra of the four specific optical links Ei → So, Si → Wo, Wi → No and Ni → Eo. The transmission spectra show that the working wavelength is near 1547 nm.

The microring can manipulate eight wavelength channels in the C band because it has a radius of 20μm, which is equivalent to an FSR of around 5 nm. The extinction ratios of the microring optical switches at their through and drop ports are approximately 27 and 15 dB. The drop port's 3-dB bandwidth is 0.59 nm (about 73.8 GHz), which is sufficient for 40 Gbps data transmission. The resonance wavelengths of the eight microring optical switches are a little separated due to a manufacturing error, which could be adjusted by using thermo-optic tuning at the expense of increased power consumption.

Tunable voltage sources are applied to change the resonance wavelengths of the mircorings to measure the optical signal-to-noise ratio (SNR) of the optical router.Fig. 5 displays the optical router's spectral responses of its three routing stages. For routing state 1, all microring optical switches are off-resonance at the working wavelength and four optical links Ei → So, Si → Wo, Wi → No, and Ni → Eo are established. For example, the purple line in the first graph represents the spectral response of the optical link Ni → Eo and the rest three lines represent the spectral responses of the optical links Ei → Eo, Si → Eo, and Wi → Eo, which are noises to the optical link Ni → Eo, as they are not formed in routing state 1. The average optical SNRs of the optical router exceed 13.8 dB. Improving the extinction ratios of the microrings and the optical crosstalks of the waveguide crossings may further increase the optical SNRs of the router.

A tunable laser and a Mach-Zehnder modulator are employed to provide external data to verify the functionality of data transmission. A pulse pattern generator generates a 40 Gbps 2³¹–1 pseudo-random bit sequence (PRBS). After being amplified by an erbium-doped fiber amplifier, the output optical signals are sent to a digital communication analyzer (DCA) for viewing the eye diagrams (seeFig. 6). It should be noted that all eye diagrams are captured with a single channel data transmission. The extinction ratio of the optical signal is 18.2 dB for back-to-back data transmission and about 16.4 dB for data transmission through the device. For the 12 optical links of the four-port optical router, open and clear eye diagrams validate the 40 Gbps the data transmission function.

As aforementioned, the microring optical switch has a periodic spectral response. As a result, wavelength division multiplexing (WDM) technology can be used to raise the aggregate bandwidth of microring optical switches based four-port optical router without modifying the routing algorithm or expending additional electrical power. WDM data transmission for the optical router is performed to validate this property. For simplicity, the result on a typical optical link Ei → So is shown inFig. 7. Eight optical signals at various wavelengths with interval equals to the FSR of the microring optical switches can be multiplexed in the whole C band. Because of the dispersion of silicon waveguide, the adjacent channel spacing is slightly different. The WDM data transmission of the optical router is validated by the open and clear eye diagrams.

The power consumption of the eight microring optical switches in the “bar” and “cross” states are measured. The switch elements in the “cross” state do not consume electrical power. The power consumption of the switches ( S1,S2,⋯,S8) in the “bar” state are 12.2, 10.6, 11.8, 12.4, 11.3, 14.0, 13.2, and 12.5 mW, correspondingly. With the assumption that every routing state appears with the same chance, the device’s average power consumption is 32.67 mW/routing state. For each optical link, the communication capacity is 320 Gbps, and the optical router can manage four optical links at the same time. As a result, the average energy efficiency of the device is 25.52 fJ/bit. We use a 10 kHz square wave to tune the state of the optical switch and test its response time. Note that the rising and falling time are defined for the through port of the microring optical switch. The 90% to 10% falling time of each optical switch fluctuates from 13 to 17µs and the 10% to 90% rising time fluctuates from 2 to 9µs.

In summary, we have demonstrated a strict non-blocking four-port optical router composed of eight microring optical switches. Furthermore, 8 × 40 Gbps data transmission has been achieved over all its 12 optical links in the whole C band. All 12 optical links in the characterized routing states have optical SNRs greater than 13.8 dB.

This work was supported by National Key Research and Development Program of China (2019YFB2203602) and National Science Fund for Distinguished Young Scholars (61825504).